8.7 - 8.10

Question 1

Note 1 —-»

Answer 1

For a plant or an animal to grow, develop normally, and maintain its tissues once fully grown, the timing of cell division in different parts of its body must be carefully controlled. For example, your skin cells and the cells lining your digestive tract divide frequently, replacing cells that are constantly being abraded and sloughed off. In contrast, cells in your liver usually do not divide unless the liver is damaged. Other cells, such as your muscle cells, never divide. By growing animal cells in culture—that is, in a laboratory growth medium, outside of the body—researchers have been able to identify many factors, both chemical and physical, that influence cell division. For example, most animal cells exhibit anchorage dependence; they must be in contact with a solid surface—such as the inside of a culture dish or the extracellular matrix of a tissue—to divide. Another physical factor that can regulate growth rate is density-dependent inhibition, a phenomenon in which crowded cells stop dividing. Animal cells growing on the surface of a dish multiply to form a single layer and usually stop dividing when they touch one another. If some cells are removed, those bordering the open space begin dividing again and continue until the vacancy is filled. What actually causes the inhibition of growth? Studies of cultured cells suggest that physical contact of cell-surface proteins between adjacent cells is responsible for inhibiting cell division.

Question 2

Anchorage Dependence

Answer 2

The requirement that to divide, a cell must be attached to a solid surface.

Question 3

Density-dependent inhibition

Answer 3

The ceasing of cell division when cells touch one another.

Question 4

Note 2 —-»

Answer 4

Chemical factors can also influence the rate of cell growth. For example, when grown in the laboratory, cells fail to divide if an essential nutrient is left out of the culture medium. Additionally, most types of mammalian cells will divide only if certain proteins, called growth factors, are present.

Question 5

Note 3 —-»

Answer 5

A protein called vascular endothelial growth factor (VEGF) stimulates the growth of new blood vessels during fetal development and after injury.

Question 6

Growth Factors

Answer 6

A protein secreted by certain body cells that stimulate other cells to divide.

Question 7

Note 4 —-»

Answer 7

The importance of proper cell division to your own health becomes clear when you consider what happens if control is lost. Cancerous cells are different from normal body cells because they no longer exhibit the types of regulation discussed here. For example, cancer cells are not subject to anchorage dependence; they grow whether or not they are in contact with a suitable surface. Additionally, density-dependent inhibition fails in tumors; cancer cells continue to divide even at high densities, piling up on one another. The importance of proper cell division to your own health becomes clear when you consider what happens if control is lost. Cancerous cells are different from normal body cells because they no longer exhibit the types of regulation discussed here. For example, cancer cells are not subject to anchorage dependence; they grow whether or not they are in contact with a suitable surface. Additionally, density-dependent inhibition fails in tumors; cancer cells continue to divide even at high densities, piling up on one another.

Question 8

Compared with a control culture, the cells in an experimental culture are fewer but much larger in size when they cover the dish surface and stop growing. What is a reasonable hypothesis for this difference?

Answer 8

The experimental culture is deficient in one or more growth factors.

Question 9

Note 5 —-»

Answer 9

The reproductive behavior of cells—whether to divide or not—results from interactions among many different molecules. In a living animal, most cells are anchored in a fixed position and bathed in a solution of nutrients supplied by the blood, yet they usually do not divide unless they are signaled by other cells to do so. Growth factors are the main signals, and their role in promoting cell division leads us back to our earlier discussion of the cell cycle.

Question 10

Note 6 —-»

Answer 10

The sequential events of the cell cycle are directed by a distinct cell cycle control system, represented by the gray circle in the center of the art. The thin gray bar extending from the circle represents the current position in the cell cycle. The cell cycle control system is a set of molecules that both triggers and coordinates key events in the cell cycle. The cell cycle is not like a row of falling dominoes, with each event causing the next one in line. During mitosis, for example, metaphase does not automatically lead to anaphase. Instead, proteins of the cell cycle control system must trigger anaphase to begin.

Question 11

Cell Cycle Control System

Answer 11

A cyclically operating set of proteins that triggers and coordinates events in the eukaryotic cell cycle.

Question 12

At which of the three checkpoints described in this module do the chromosomes exist as duplicated sister chromatids?

Answer 12

G2 and M checkpoints

Question 13

Note 7 —-»

Answer 13

Cancer, which claims the lives of one out of every five people in the United States, is a disease of the cell cycle. Cancer cells do not heed the normal signals that regulate the cell cycle; they divide excessively and invade other tissues of the body. If unchecked, cancer cells may continue to grow and spread until they kill the organism. Cancer begins when a single cell undergoes changes that convert a normal cell to a cancer cell. Such a cell often has altered proteins on its surface and the body’s immune system normally recognizes the cell as an alien and destroys it. However, if the cell evades destruction, it may multiply to form a tumor, a mass of abnormally growing cells within otherwise normal tissue. If the abnormal cells remain at their original site, the lump is called a benign tumor. Benign tumors can cause problems if they grow in and disrupt certain organs, such as the brain, but often they can be completely removed by surgery or even (in cases in which they pose no imminent threat) left alone.

Question 14

Note 8 —-»

Answer 14

In contrast, a malignant tumor is a mass of abnormally reproducing cells that can spread into neighboring tissues and invade other parts of the body, with the potential to displace normal tissue and interrupt organ function as it grows. An individual with a malignant tumor is said to have cancer. Cancer cells may separate from the original tumor or secrete signal molecules that cause blood vessels to grow toward the tumor. A few tumor cells may then enter the blood and lymph vessels and thereby move to other parts of the body, where they may proliferate and form new tumors. The spread of cancer cells beyond their original site is called metastasis.

Question 15

Tumor

Answer 15

Abnormal mass of rapidly growing cells that forms within otherwise normal tissue.

Question 16

Malignant Tumor

Answer 16

An abnormal tissue mass that can spread into neighboring tissue and to other parts of the body.

Question 17

Cancer

Answer 17

A disease characterized by the presence of malignant tumors in the body.

Question 18

Metastasis

Answer 18

The spread of cancer cells beyond their original state.

Question 19

Note 9 —-»

Answer 19

Cancers are named according to the organ or tissue in which they originate. Liver cancer, for example, starts in liver tissue and may or may not spread from there. Carcinomas are cancers that originate in the external or internal coverings of the body, such as the skin or the lining of the intestine. Leukemia is a broad term covering a number of diseases that originate in immature white blood cells within the blood or bone marrow. Many cancer cells have defective cell cycle control systems that proceed past checkpoints even in the absence of growth factors. Other cancer cells synthesize growth factors themselves, causing the cells to divide continuously. If cancer cells do stop dividing, they seem to do so at random points in the cell cycle rather than at the normal cell cycle checkpoints. Moreover, in the laboratory, cancer cells are “immortal”; they can go on dividing indefinitely, as long as they have a supply of nutrients (whereas normal mammalian cells divide only about 20 to 50 times before they stop). A striking example of the immortality of cancer cells is a line that has been continuously multiplying in culture since 1951. Cells of this line are called HeLa cells, named for the original donor, Henrietta Lacks, who died of cervical cancer more than 65 years ago.

Question 20

Note 10 —-»

Answer 20

A tumor that appears to be localized may be removed surgically. Alternatively, it can be treated with concentrated beams of high-energy radiation, which usually damages DNA in cancer cells more than it does in normal cells, perhaps because cancer cells have lost the ability to repair such damage. However, radiation also damages normal body cells, producing harmful side effects. For example, radiation damage to cells of the ovaries or testes can lead to sterility. Chemotherapy is used to treat widespread or metastatic tumors. During periodic chemotherapy treatments, intravenous (IV) drugs are administered that disrupt specific steps in the cell cycle. For instance, the drug Taxol freezes the mitotic spindle after it forms, which stops actively dividing cells from proceeding past metaphase. Vinblastin, a chemotherapeutic drug first obtained from the periwinkle plant, prevents the mitotic spindle from forming in the first place. The side effects of chemotherapy are due to the drugs’ effects on cells that rapidly divide in parts of the body beyond those where the cancer resides. Nausea results from chemotherapy’s effects on intestinal cells; hair loss comes from effects on hair follicle cells, and susceptibility to infection results from effects on immune cell production.

Question 21

What is metastasis?

Answer 21

Metastasis is the spread of cancer cells from their original site of formation to other sites in the body.

Question 22

Note 11 —-»

Answer 22

Oncologists (doctors who treat cancer) have observed that different cancer patients respond in drastically different ways to the same treatment: Therapies that are effective for some patients may be unhelpful or even harmful for others. Treatment options for cancer are therefore changing from a “one-size-fits-all” model to a determination of the best therapies for each particular patient. In 2015, researchers from Toronto published the most extensive analysis of data ever conducted on the treatment of DCIS (ductal carcinoma in situ, also called stage 0 breast cancer). Unlike more insidious forms of cancer, the progression of DCIS is uncertain: the abnormal cells might go away on their own, they might remain in place and never cause harm, or they might grow and spread, possibly leading to death. The 2015 study followed 108,000 American women for 20 years after diagnosis with DCIS. Death rates from breast cancer were examined and broken down by such factors as age at diagnosis, ethnicity, and socioeconomic status. This is an example of an observational study, one that draws inferences from a data set in the absence of controlled experiments. Such studies may be undertaken to help determine the possible benefit of a treatment on research participants with whom the use of controlled groups would be impractical. Oncologists emphasize that additional research must be undertaken before any widespread changes can be made in DCIS treatment protocols.

Question 23

Why must human cancer research often use an observational method when controlled studies could yield more definitive results?

Answer 23

It is clearly unethical to force cancer-causing behaviors on test subjects, so researchers can only observe what behaviors occurred during the study and how these behaviors correlated with outcomes.